Inverted Tee Cap Flexure (AASHTO LRFD)

For flexure design of inverted Tee cap at each location, program considers the maximum positive and negative moments in design. The required steel area for each section is computed based on the maximum positive and negative moments. Depending on the neutral axis location, program considers the rectangular section or flanged Tee section.

This required steel is then checked for minimum steel criteria of Art. 5.7.3.3.2. Substructure computes temperature and shrinkage steel as per LRFD Art. 5.10.8.2.

Ledge Reinforcement

To calculate Ledge reinforcement, for interior beams, program calculates distribution of shear and flexure as minimum of:

W + 4av

And for exterior as 2C, where w is bearing width, av is distance of center of bearing from the face of stem, S is spacing between bearings and c is edge distance of last bearing. At each bearing location, the program finds out the max shear value. The program calculates the reinforcement required for shear using AASHTO LRFD 5.13.2.4.2. It calculates ledge reinforcement for flexure program performs the iterative method to calculate neutral axis location and moment capacity. The program checks the ledge reinforcement for axial tension, based on AASHTO LRFD 5.13.2.4.2.

For M bars, program checks the reinforcement area using AASHTO LRFD 5.13.2.4.1, 5.13.2.4.2. For N bars, program checks the min. reinforcement as per LRFD Eq 5.1.3.2.4.2-6.

Punching Shear Check

Program checks, if shear cones are intersecting using:

\frac{1}{2} S - \frac{1}{2} W \geq d_{f}

\frac{1}{2} b_{s t e m} + a_{v} - \frac{1}{2} L \geq d_{f}

If any one of the checks fails, the program flags the appropriate location and recommends you to revise ledge dimensions. For an interior node, program checks if

V_{u} < ϕ \times 0.125 \sqrt{{f^{'}}_{c}} (w + 2 L + 2 d_{f}) d_{f}

At any node, if Vu< ϕVn, the program flags the locations. The program designs hanger reinforcement, for both shear and torsion. The min. transverse reinforcement is checked using LRFD 5.8.2.5-1

Even when sections are deeper than 48 inches, the program first checks if the section is adequate for the applied moment from the critical combination. It then checks if the design moment is at least equal to the minimum moment. This has to be at least equal to the smaller of 1.33 Mu or 1.2 Mcr. You can manually input the modulus of rupture, fr, value in A/D parameters under Analysis tab. By default, the program uses for classic approach and when ϕ as per LRFD is selected. If needed, revised steel is calculated. Then the program checks for temperate and shrinkage steel. When classic phi approach is used, shrinkage steel is equal to 0.11×Ag/Fy. However, the code specifies to distribute this in two faces. The program internally checks separately for top and bottom steel. It checks if the top and bottom steel is at least equal to 0.5(0.11×Ag/Fy) individually. When phi as per LRFD 2006 is used, program uses the revised criteria of bh / 2 (b+h) fy as specified in LRFD 2006 interims. In certain cases, it is possible that there is no moment in a section. In such a case, zero moment might be reported for combination number 0. However, temperature and shrinkage steel would still be required. Therefore, the program in LRFD mode will show required steel based on temperature and shrinkage.

For capacity calculation, program first computes effective steel area. This effective area is obtained after adjusting the gross steel area and taking development length into account. The beam cross section capacity (ϕMn) at every location is then computed using strain compatibility with consideration of concrete material, and any available top and bottom bars.

The calculation of φ for flexure design for cap and strut in AASHTO LRFD specification is done in two possible ways. User can choose to do the design as per classic approach (LRFD 2005 interims and earlier) or as per LRFD 2006 approach. This choice can be made on the Analysis / Design Parameters (A/D) screen on Analysis tab. Please see the section Resistance Factor/Strength Reduction Factor φ for more information.

When the classic design (LRFD 2005 interims and earlier) approach is selected, the maximum reinforcement ratio as per LRFD 5.7.3.3.1 is checked. If the c/d value for a section is greater than 0.42, section is said to be over-reinforced and is flagged.